Could he be cheating on me?

I've been in a long distance relationship for four months now and we have managed to make it work and see each other every few weeks. But when we are apart we hardly talk, there are hours between his replies to my texts. Also if we are on the phone normally at the end of the conversation he says love you but doesn't when he's around his friends. I hate that I'm acting paranoid but it's because of past relationships and I don't know how to stop it.